Self-powered neutron detectors are known, as exemplified by the U.S. Treinen et al. U.S. Pat. No. 3,390,270, dated June 25, 1968.
To provide such a detector with a service life long enough to permit its practical use in the internal instrumentation of a reactor core, such as a pressurized-water reactor core, the Klar U.S. application Ser. No. 306,940, filed Nov. 15, 1972, discloses the use of thulium 169 as the emitter material. Generally speaking, suitable emitter materials are also sensitive to gamma radiation so that a falsified signal may result, and to compensate for gamma radiation sensitivity, the Klar et al. U.S. application Ser. No. 386,439, filed Aug. 7, 1973, discloses the type of detector, such as disclosed by the Treinen et al patent, provided with a second emitter material having a gamma sensitivity but which is substantially insensitive to neutron flux, whereby the detector produces two outputs which by suitable instrumentation provides a signal which is gamma compensated.
The provision of a detector of the Treinen et al patent type, but using thulium 169 as the emitter material, is desirable because of its simplicity, providing the detector can be made so that it avoids a falsified signal due not only to gamma radiation, but also because of interference signals based on secondary effects which are time-delayed.